Equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment fair value changes, net									$ (28,574)	$ (21,357)	$ (10,202)
Realized gains, net									28,009	36,369	15,478
Interest expense	$ 20,537	$ 21,597	$ 22,444	$ 23,950	$ 25,039	$ 23,875	$ 23,008	$ 23,961	88,528	95,883	87,463
Net income before provision for income taxes									134,960	91,742	$ 101,313
Other than temporary impairments									3,000	400
Other than temporary impairment losses, income statement									400	200
Accumulated other comprehensive income, other-than-temporary impairments									3,000	200
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Gains on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment fair value changes, net									368	246
Realized gains, net									(21,535)	(29,672)
Net income before provision for income taxes									(21,167)	(29,426)
Reclassification out of Accumulated Other Comprehensive Income | Net Unrealized Losses on Interest Rate Agreements Accounted for as Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense									72	95
Net income before provision for income taxes									$ 72	$ 95